macro-safety-do-while-false-idiom.md

Macro Safety: The do-while-false Idiom

It's common to wrap multi-statement macros in a do { ... } while(false) block. This is done to make the macro safe when used inside control structures like if without braces; without it, macros that expand to multiple statements can behave in unexpected ways.

For example, say I define a macro like this:

#define my_macro do_something(); do_something_else();

And then I use it like this:

if (condition)
  my_macro;

That expands to:

if (condition)
  do_something();
do_something_else();  // ← this part is **not** inside the if block!

This leads to a bug, since do_something_else() always runs, regardless of the condition.

By wrapping the macro like this:

#define my_macro do { do_something(); do_something_else(); } while (false)

Now when it's used:

if (something)
  my_macro;

It expands to:

if (something)
  do { do_something(); do_something_else(); } while (false);

This behaves as expected: everything stays inside the if.

The do { ... } while(false) pattern makes macros act like a single statement!